The invention is generally related to floating offshore structures and more particularly to semi-submersible floating offshore structures.
The semi-submersible is a type of floating structure that has vertical columns supporting topsides and supported on large pontoons. The structure is held in position by the use of spread mooring lines that are anchored to the seafloor. The semi-submersible has a number of unique characteristics compared with other floating structures such as a spar and TLP (tension leg platform). These advantages include: The semi-submersible has good stability because of a large footprint and low center of gravity for the topsides. The hull requires lower steel tonnage. The hull can be a new build or converted from an existing drilling semi. The semi-submersible may include drilling capability. The semi-submersible can support a large number of flexible risers or SCRs (steel catenary risers) because of the space available on the pontoons. The topsides can be integrated at quayside and thus reduce cost and save scheduling time. The semi-submersible has a relatively short to medium development schedule. The initial investment is relatively low.
The semi-submersible also has several deficiencies when compared with the spar and TLP. The most significant is the large heave motion because of the shallower draft and large pontoons. As a result, it has not been suitable for a dry tree riser arrangement. The dry tree riser arrangement has significant economic benefit for well completion, work-over, and intervention during the life of the offshore production facility. Another problem from the large motion of the semi-submersible is that it causes fatigue in the SCRs more easily, which requires more stringent fatigue design for the SCRs and thus costs more. For a platform in ultra deepwater with large diameter SCRs, the solutions to this problem could become technically or economically unfeasible.
The ideas that have been explored by the industry to overcome the semi-submersible motion problem generally fall into the two categories below.
The first is a deep draft semi-submersible. The concept is to increase the draft from the normal range from sixty to eighty feet to one hundred to one hundred ten feet so that the wave action at the keel is reduced and, thus, the structure will have less motion. This makes the semi-submersible option feasible in some locations where the conventional semi-submersible would not be chosen because of the difficulties in dealing with the SCR riser fatigue issues. However, the heave motion is still relatively large compared with spars and TLPs. Also, the dry tree arrangement is still not feasible. The SCRs deployed on the deep draft semi-submersibles usually still need to be strengthened to meet the fatigue life requirement.
The second is a semi-submersible with a heave plate. The basic idea is to add a heave plate or pontoon at the keel that extends in deep draft. The heave plate or pontoon adds damping and added mass to the platform which will reduce its heave motion under wave conditions.
Most concepts based on the heave plate have the heave plate or pontoon as an extendable part at the bottom of the semi-submersible hull. The heave plate or pontoon is retracted at the fabrication yard and during transportation. After the hull is located on site, the heave plate or pontoon is then extended or lowered to a deeper elevation and locked at that position.
The known designs suffer several deficiencies. The hull has to be a new build and conversion of an existing semi-submersible hull is not possible. The extendable columns take too much deck space. In some cases it could be as much as thirty percent of the total deck space, which is impractical from a topsides equipment layout point of view. The column-to-deck connections are complicated. They are hard to build, risky during installation, and difficult to maintain. The advantage of a large pontoon area for riser supports from the conventional semi-submersible hull is compromised.